If you like smartphone free days, or just in the car or riding your bike, we found a great solution.

These GSM watches are available as replacement for smartphones at very low-cost. Model names are G36 and Q50.

Features are

speakerphone

basic calls

Internet based time

You can add them as classical extensions on your PBX. A parallel call will ring them together with your SIP phone. Your trunk's caller ID should be added as a contact number to the watch, to make the watch whitelist all forwarded incoming calls.

Watches can be configured easily via an Android app ("Setracker"). An additional SIM card with low-bandwidth always-on Internet is required.